1. Field of the Invention
The present invention relates to a hydraulic tensioner for imparting appropriate tension to a chain, belt or the like, and more particularly, to a hydraulic tensioner having a ratchet mechanism to prevent retraction of a piston at the time of decreased hydraulic pressure.
2. Description of Related Art
A hydraulic tensioner generally includes a housing, a piston that is biased in a protruding direction by a spring and fitted slidably into a piston hole formed in the housing, and a fluid chamber defined by the piston and the piston hole of the housing. During operation, the combined efforts of the spring load and the hydraulic pressure in the chamber balances the external force from a chain or belt imparted on the distal end of the piston.
When external force is applied from a chain to the distal end of a piston during inadequate hydraulic pressure in the chamber, such as at the time of engine start, the piston is easily forced to retract into the housing, thereby causing noise or oscillation.
In order to prevent such retraction of the piston, various kinds of hydraulic tensioners with ratchet mechanisms have been proposed, such as shown in Japanese patent application laid-open publication No. 7-158703.
The hydraulic tensioner of Japanese patent application laid-open publication No. 7-158703 includes a rack received in a longitudinal hole disposed parallel to the piston hole and engaged with a shoulder portion of the piston via a flange, a ratchet received in a lateral hole communicating with the longitudinal hole and engaged with the rack, and a spring biasing the ratchet in an engaging direction with the ratchet.
When the piston protrudes, the rack moves along with the piston via the flange in a protruding direction. When the piston retracts due to the external force of the distal end of the piston of the chain, only the piston retracts and as the distal end of the piston moves to the position of the distal end of the rack, the rearward movement of the piston is halted by the engagement of the ratchet with the rack. Additionally, in this case, a stepped portion of the distal end of the piston relative to the distal end of the flange provides backlash for the tensioner.
Another example of a hydraulic tensioner with a pawl ratchet mechanism is U.S. Pat. No. 5,304,099. The ratchet mechanism includes a rack teeth portion that is formed on a sleeve disposed around the piston, a pawl housed in a pawl hole extending perpendicular to a piston bore with a distal end portion engageable with the rack teeth portion of the sleeve, and a pawl coil spring that biases the pawl in an engaging direction with the rack teeth portion.
When the piston moves in a protruding direction along the sleeve, the distal end portion of the pawl engages the rack teeth portion of the sleeve, and the piston is allowed to move in the protruding direction in such a way that the rack teeth portion of the sleeve travels over the distal end portion of the pawl. When the piston moves in a retracting direction along the sleeve, the piston is prevented from moving in the retracting direction by maintaining the engagement of the rack teeth portion of the sleeve with the distal end portion of the pawl.
The prior art tensioners require a rack separate from the piston, thereby making the structure of the tensioners complicated.
The present invention addresses such problems and its object is to provide a hydraulic tensioner with a backlash mechanism that can simplify the structure of the tensioner. Another object of the present invention is to provide a hydraulic tensioner with a backlash mechanism that can apply stable spring force to the pawl member.